docs: def df_representative_structures(self):
          """DataFrame: Get a dataframe of representative protein structure information."""
          rep_struct_pre_df = []
          df_cols = ['gene', 'id', 'is_experimental', 'file_type', 'structure_file']
  
          for g in self.genes_with_a_representative_structure:
              repdict = g.protein.representative_structure.get_dict(df_format=True, only_attributes=df_cols)
              repdict['gene'] = g.id
              rep_struct_pre_df.append(repdict)
  
          df = pd.DataFrame.from_records(rep_struct_pre_df, columns=df_cols).set_index('gene')
          if df.empty:
              log.warning('Empty dataframe')
              return df
          else:
              return ssbio.utils.clean_df(df)